1. Field of the Invention
The present invention relates to a video projection device, and more specifically to a video projection device (for example, projector for digital cinema) having a converter lens attachably and detachably provided for a projection lens.
2. Description of Related Art
For movies, it is required to reproduce and express movie creator's intention including their tone as faithfully as possible. Thus, in recent digital cinema, various regulations related to chromaticity etc. on a projection system therefor have been defined. On the other hand, in theaters, a converter lens is attached to a projection lens in order to deal with various image plane formats (for example, see Patent Document 1).
Patent Document 1: U.S. Pat. No. 6,587,159
In a large projection system, for example, the one for projecting a movie, a converter lens used is also large. Moreover, a transmittance characteristic, etc. of an entire projection optical system also change under the influence of absorption by an optical material used in the converter lens, reflection on a lens interface, etc. Thus, it is a challenge to satisfy the aforementioned regulations even when the converter lens is attached. However, a conventional movie projection system as described in Patent Document 1 is basically designed for film projection and thus is not designed to correct a change in video characteristics caused by attaching the converter lens.
A projector for film projection has difficulties in correcting the change in the video characteristics caused by attaching the converter lens. This is because, for example, a color filter needs to be added to an illumination optical system to change the tone and an aperture mechanism needs to be provided to change the brightness. This results in formation of a massive device as a whole, leading to cost increase. Providing a color filter in the converter lens in particular is not preferable due to concern about interplanar ghost. Changing the projection position requires movement of a physically large projection lens, which results in formation of a massive device as a whole, leading to cost increase. Under these circumstances, for a projector for digital projection, no efforts have yet been made to improve video characteristics that changes as a result of attaching the converter lens.